The Permethians

A few Permethians escaped out into the depths outside the galaxy, the darkest space. They were going to sit there until they could reclaim their mantle and the forerunners eventually kind of petered out. The only way to get rid of this spore that would spread the humans, make them want to be carriers and spreaders themselves was to kill off all those infected people. And in 100 years, that spore began to spread the people. That spore was what was left of the comrithians and their weird cells and it became the flood. But then the forerunners had awakened to the anger state that they became and a conquered humans. We really didn't stand a chance